This invention relates to transmission of speech-band telecommunications signals such as voice and facsimile signals over a power line communications system.
Voice and data transmission is conventionally conducted between subscriber terminals over a PSTN, with the local subscriber loop being provided by twisted wire pairs.
There have been a number of proposals to carry voice and data signals over the limited distances of electrical wiring within a building, and to perform meter reading and load control applications at low bit rates on an electricity distribution network external to buildings. Until recently the distribution network external to a building has not been used to carry voice or high bit rate data communications.
International Patent Applications WO 95/29537 A1 (Norweb) and WO 98/06188 A1 (Northern Telecom) describe electricity distribution networks which carry telecommunications signals. Delivering a telecommunications service in this manner is attractive as it overcomes the need for installing cabling to each subscriber, which is one of the greatest costs in providing a new telecommunications network. Existing power lines are used to carry the telecommunications signals to subscribers.
International Patent Application WO 95/29537 describes the use of existing circuit-switched telephony standards such as CT2 to carry telecommunications signals over a power line network. Standard CT2 equipment is used at subscriber terminals and the output of this equipment, which lies in the frequency band around 866MHz, is converted to a more acceptable frequency band for transmission over the power line network.
Power line transmission is one of several ways for delivering communications to subscriber premises and is therefore sensitive to competition with existing copper wires and more recent alternatives such as optical fibre/coaxial cable delivery and fixed radio access techniques. Therefore, there is a desire to provide a system that is capable of delivering an acceptable quality of service at a cost that is attractive to a subscriber.
The present invention seeks to provide an alternative way of carrying speech-band signals, such as voice signals, over a power line communications system.
A first aspect of the present invention provides a method of transmitting speech-band signals over a power line communications system comprising a power line for distributing electricity to a plurality of premises and a communication station coupled to the power line, the method comprising:
receiving speech-band signals at the communications station from speech-band communications equipment;
digitising the speech-band signals;
encoding the digitised signals into data packets;
transmitting the data packets to/from the station over a part of the line which is external to the premises, using a packet data communications protocol.
Transmitting the speech-band signals in digitised form using a packet data protocol allows the speech to be carried in a robust form over the power line. Packet length and timing of the packets can be tailored to the power line environment. It also allows a mix of speech and data from a number of different subscribers to be carried over a common channel in a packet-switched manner, thereby making much more efficient use of the available spectrum on the power line.
Transmitting the speech-band signals over the power line in the form of data packets can have an advantage that premises on the power line need not be xe2x80x98conditioned.xe2x80x99 The term conditioned means that premises are fitted with low-pass filter units to prevent noise from premises entering the power line and corrupting communications signals. Avoiding the need to condition the network significantly reduces the cost of setting up a power line communications system.
The term xe2x80x98speech-bandxe2x80x99 signals is intended to cover voice telephony, facsimile and other speech-band signals that would traditionally be carried over a PSTN.
The packet data protocol is preferably a polled protocol in which communications stations on the power line are polled in turn to allow them to access the shared resource of the frequency band(s) used for communications over the power line. Other protocols can also be used, such as token-passing or CSMA. Each communications station has an address and the data packet includes an address which identifies the intended recipient station of the packet.
The speech-band signals can be encoded into a form in which they are carried over the power line or they may also be encoded into another form, such as internet protocol (IP), as well as being encoded into the packet data communications protocol that is used on the power line. This is particularly useful where the called party is a person who is using a computer terminal coupled to the internet to make a telephone call.
The power line can comprise underground or overhead lines or a combination of these.
Preferred features may be combined as appropriate, and may be combined with any of the aspects of the invention as would be apparent to a person skilled in the art.